When changing the cartridge of a refillable injection device it is desirable to let the piston of the new cartridge push the piston rod of the injection device back into the injection device. Thereby it is ensured that the piston rod contacts the piston when the cartridge is positioned correctly, and the injection device is therefore ready for use. Thereby the need for very large air shots before taking the injection device into use is avoided.
However, in prior art refillable injection devices having a dose rod, the piston rod is pushed, pulled or rotated back manually, thereby introducing the problem that it can not be ensured that the piston rod contacts the piston, and very large air shots are therefore necessary in order to ensure correct use of the injection device.
In order to ensure proper operation of the injection device during setting of a desired dose, the dose rod and a dose setting member must be rotationally locked when the cartridge is positioned correctly. This is usually obtained by letting the dose rod and the dose setting member engage, e.g. by means of mating sets of teeth positioned on the dose rod and the dose setting member, respectively. This may cause stress to be introduced in the piston rod, and this tension may have the consequence that liquid drug will be expelled from the injection device when a needle is subsequently attached to the new cartridge.
In WO 02/092153 an attempt to solve the above mentioned problem is disclosed. Thus, WO 02/092153 discloses a medication injector apparatus, such as an injection pen. The injection pen includes a resettable cartridge plunger drive assembly including an axially floating nut, a cartridge plunger engaging screw, and a drive clutch movable with the nut and which when rotated causes the screw to screw through the nut. When a cartridge assembly is mounted to the pen base, the floating nut and drive clutch are shifted proximally such that the drive clutch is in torque transmitting engagement with a rotatable drive member of the pen, such that rotation of that drive member results in drive screw advancement through the nut in the distal direction. When the cartridge assembly is not mounted to the pen base, the floating nut and drive clutch are biased distally to disengage the drive clutch from torque transmitting engagement with the rotatable drive member and to thereby allow the drive screw to be reset proximally through the nut to a position more retracted within the pen base. Thereby drooling of the pen is limited.
The injection device of WO 02/092153 is not of the kind having a dose setting mechanism comprising a dose rod, and the mechanism disclosed therein is not suitable for use in such an injection device.